This invention relates generally to well blowout preventers, and more particularly concerns packer units used in such equipment.
For many years, the design of blowout preventer packing units has followed the principles described in U.S. Pat. No. 2,609,836 to Knox. Such units incorporate like metal inserts equally spaced about the packer central axis, and embedded by an elastomeric body. Upon inward constriction or closure of the unit about a well drill pipe, the material is anchored by insert webs as it produces vertical folds stretching radially inwardly to seal against the pipe. In general, the number of folds will equal the number of inserts, and they will be alike in circumferential contour. When the packer unit insert close on itself, with no pipe present, the elastomeric material of the folds advancing toward the axis must at certain times and places stretch or extend as much as 350 to 400%. Repeated closures produce excessive wear and fatigue of the elastomeric or rubber material, reducing the useful life of the packer due to such extreme stretching. Also, the rubber quality must be extremely closely controlled to ensure successful closure and seal-in thousands of pounds per square inch of well fluid pressure. Accordingly, there is a need for a packing unit characterized by significantly reduced rubber stretching, and the useful life of which will be extended over many more closures than conventionally possible.
Another problem with packer design has to do with damage to the rubber that tends to flow or extrude into the spaces between like end-plates on the insert webs, as the plates move relatively inwardly and toward one another during packer constriction.
An improved packer unit is described in U.S. Pat. No. 3,917,293 to Lewis et al. That patent discloses, among other things, the use of alternately different web plates to control packer constriction.